With an increase in the data processing properties of portable devices, more information can be stored in them, which may also be confidential or otherwise such information that must not be revealed to an outsider. The carrying of portable devices will, however, increase the risk that the portable device is lost or stolen, wherein an attempt must be made to protect the information stored in it with an encryption method. For portable devices, it is typically possible to determine a password which the user must enter in the device when the device is turned on before the device can be used normally. However, such a protection is relatively easy to pass, because the passwords that are used are normally relatively short, typically having a length of less than ten characters. On the other hand, even if no attempt were made to find out the password, the information contained in the device can be accessed for example by transferring the storage medium, such as a hard disk, into another device. If the information contained in the storage medium is not in encrypted format, the information stored in the storage medium can be easily found out.
It is known that information needed by the user or the device can be encrypted with one key, the encrypted information can be stored in the memory of the device, and it can be decrypted with another key. In asymmetric encryption, the key used in encryption is different from the key used in decryption. Correspondingly, in symmetric encryption, the key used in encryption is the same as the key used in decryption. In asymmetric encryption, these keys are normally called a public key and a private key. The public key is intended for encryption and the private key is intended for decryption. Although the public key may be commonly known, on the basis of the same it is normally not possible to easily determine the encryption key corresponding to the public key, wherein it is very difficult for an outsider to find out information encrypted with this public key. One example of a system based on the use of such a public key and a private key is the PGP system (Pretty Good Privacy), in which the user encrypts the information to be transmitted with the public key of the receiver, and the receiver will then open the encrypted information with his/her private key. However, there are considerable drawbacks in the systems of prior art. The key strings required by sufficiently secure systems are so long that even their storage in a safe way causes considerable costs. If the key string is too short, it will be relatively easy to break it up with modern data processing equipment. In other words, the private key can be defined on the basis of the content and the public key (known content attack). This problem is particularly significant in portable data processing and communicating devices, in which the limited processing capacity also prevents the use of long keys.
The U.S. Pat. No. 6,169,890 discloses a method and device in which, after the user has been identified, it is possible to use a key stored in a SIM (Subscriber Identity Module) card for user identification in a communication network. The system is intended to be used for example in payment transactions, wherein the user performs payment transactions in his/her terminal which is arranged to communicate with a terminal of the payment system by means of the mobile communication network. Thus, the user enters his/her PIN (Personal Identity Number) code in the mobile station, and the user of the mobile station is identified on the basis of the code. The system comprises a database in which the mobile phone numbers of the users authorized to use the system have been stored. Thus, when the user contacts such a system, it is checked from the database on the basis of the number of the caller, whether the user is authorized to use the service. One drawback of such a system is that the PIN code used in the identification of the user is relatively short, typically four characters long, wherein it is relatively easy to find it out by means of methods known at present. When the length of the key is increased, the amount of memory required in the SIM card for storing the PIN code should be increased as well, which considerably raises the manufacturing costs of the SIM card. On the other hand, the act of storing the private key to such a memory which can be examined from outside the device, may present a significant safety risk, because when the device is lost or when it otherwise ends up in the hands of outsiders, the encrypted information may be found out on the basis of the encryption key stored in the device.